Materials for packaging foods and various articles often are required to have a gas barrier property, particularly, an oxygen barrier property. This is intended to preclude effects such as oxidation degradation of packaged contents that is caused due to oxygen, for example. Particularly, with respect to food packages, the presence of oxygen allows microorganisms to proliferate and thereby the contents decays, which is a problem. Hence, in conventional packaging materials, gas barrier layers that prevent oxygen from permeating therethrough are provided, so that the permeation of oxygen is prevented, for example.
Such a gas barrier layer can be, for example, a vapor deposition layer of metallic foil, metal, or a metal compound. Generally, aluminum foil, an aluminum vapor deposition layer, a silicon oxide vapor deposition layer, an aluminum oxide vapor deposition layer, etc. are used. However, metal layers such as the aluminum vapor deposition layer and aluminum foil have disadvantages in that the packaged contents cannot be seen or disposability is low, for example. Furthermore, metal compound layers such as the silicon oxide vapor deposition layer and the aluminum oxide vapor deposition layer have disadvantages in that the gas barrier property thereof is degraded considerably when the packaging material is deformed, dropped, or subjected to an impact during transportation, for example.
Moreover, a layer formed of a vinyl-alcohol-based polymer that is excellent in gas barrier property may be used as a gas barrier layer in some cases. Examples of the vinyl-alcohol-based polymer include polyvinyl alcohol, an ethylene-vinyl alcohol copolymer, etc. Such a layer formed of a vinyl-alcohol-based polymer has advantages in being transparent and having less difficulty in disposal. Accordingly, the range of uses thereof is increasing.
The above-mentioned vinyl-alcohol-based polymer is crystallized through hydrogen bonds by which hydroxyl groups contained in the molecules thereof are bonded to each other, and thereby exhibits the gas barrier property. Hence, the conventional vinyl-alcohol-based polymer exhibits a high gas barrier property in a dry state. However, in a state where it has absorbed moisture due to, for instance, water vapor, the hydrogen bonds are loosened and thereby the gas barrier property thereof tends to deteriorate. Accordingly, it is difficult to allow a vinyl-alcohol-based polymer such as polyvinyl alcohol to exhibit a high level of gas barrier property under a high humidity condition.
Moreover, materials containing a polymer compound and a hydrolyzed and condensed product of metal alkoxide (for instance, tetramethoxysilane) have been studied as materials with a gas barrier property (for example, JP2002-326303A, JP7(1995)-118543A, and JP2000-233478A).
Recently, retort foods are being produced increasingly. The retort foods are produced by packing contents in a food packaging material and then immersing it in hot water to subject it to a sterilization treatment. In such a situation, the level of performance that is required of packaging materials for retort foods further is increasing. Examples of the performance include a high bag breaking strength when a food packaging material including contents packed therein is dropped, an oxygen barrier property after it is sterilized in hot water, an oxygen barrier property under a high humidity condition until it is delivered to a consumer, etc. Particularly, there is demand for packaging materials that exhibit a high oxygen barrier property regardless of humidity, exhibit a high oxygen barrier property even after being subjected to retort processing, and are excellent in strength and transparency. The above-mentioned conventional techniques, however, cannot satisfy such demands well.